Pall McMillan discovered that Django used the root namespace when storing
cached session data. A remote attacker could exploit this to modify
sessions. (CVE-2011-4136)

Paul McMillan discovered that Django would not timeout on arbitrary URLs
when the application used URLFields. This could be exploited by a remote
attacker to cause a denial of service via resource exhaustion.
(CVE-2011-4137)

Paul McMillan discovered that while Django would check the validity of a
URL via a HEAD request, it would instead use a GET request for the target
of a redirect. This could potentially be used to trigger arbitrary GET
requests via a crafted Location header. (CVE-2011-4138)

It was discovered that Django would sometimes use a request's HTTP Host
header to construct a full URL. A remote attacker could exploit this to
conduct host header cache poisoning attacks via a crafted request.
(CVE-2011-4139)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 11.10:
python-django 1.3-2ubuntu1.1

Ubuntu 11.04:
python-django 1.2.5-1ubuntu1.1

Ubuntu 10.10:
python-django 1.2.3-1ubuntu0.2.10.10.3

Ubuntu 10.04 LTS:
python-django 1.1.1-2ubuntu1.4

In general, a standard system update will make all the necessary changes.